The invention is directed to a vacuum oven having a gas cooling device, in which the heat treated charge is blown on with a cooling medium from nozzles arranged around it for cooling. The cooling device has the task of quickly cooling down the charge and oven again after the ending of an annealing or calcining process.
The quick cooling of a heat threated charge in a vacuum oven can be necessary for economical reasons (better oven loading) or for process engineering reasons (prescribed high cooling rate). As cooling medium in each case there is employed a gas which is circulated whereby it takes up heat from the charge and gives it up again in a cooler. Gas circulator and cooler during this process can be arranged outside the oven; however, it is also possible that the cooling surfaces and the circulating devices can be integrated into the furnace.
In principle there are two methods to lead the gas through the charge-space. The most current type is parallel flow through the heating space wherein the gas enters on one side and leaves on the other. In connection therewith there is concern to maintain constant the velocity over the cross section of the oven. These methods have the disadvantage that very large amounts of gas must be circulated in order to produce a high heat transfer number, since the gas velocity produces a decisive size for this and flow cross section for the most part are very large.
Another type of gas cooling takes place via nozzles. In this case the charge space is surrounded by numerous nozzles. The gas flows centrally into the charge chamber through these nozzles, the gas escapes from the charge chamber through leaks in the insulation or through deliberately installed openings therein, is led through a cooler and is again pushed through the nozzles from a compressor.
Compared to parallel flow these cooling methods have the advantage that the required cooling velocity can be attained with substantially smaller amounts of gas. Of course thereby a higher pressure is necessary so that the circulating capacity is about the same size in both cases. The higher pressure needed does not require additional expense of construction while the smaller amounts of gas in the nozzle cooling reduces the expense of construction considerably.
In spite of these advantages the nozzle cooling is not always usable since it normally produces nonuniform cooling results inside the charge. Deviations of more than one 100% are no rarity. Through this there arise in the charge large temperature differences, with all the negative consequences such as high internal stresses, danger of cracks and deformation.
Therefore it was the problem of the present invention to provide a vacuum oven with a gas cooling device in which the heat treated charge for cooling is blown with a cooling medium from nozzles arranged around it and which makes possible a uniform cooling of the heat treated charge.